This invention relates to a hob for generating gear teeth on a gear blank.
Hobs have been known for many years and basically comprise a thread in the form of a series of teeth which extend helically around and along a hob body such that the teeth are spaced axially and angularly from one another. The teeth thus are arranged in a plurality of generally axially extending and generally circumferentially spaced rows with the teeth of each row being spaced generally axially from each other.
Each tooth includes a leading cutting face, a pair of side flanks, an outer tip and a root. A cutting edge is defined along the junction of the cutting face with the flanks and the tip. The flanks and the tip are appropriately relieved or angled in order to provide cutting clearance for the cutting edge. High clearance angles are desirable in order to avoid excessive wear during cutting.
Hobs which are in present day commercial use are adapted to be re-sharpened at the cutting face after the cutting edge becomes worn. Thus, the teeth as originally manufactured are shaped so as to enable each tooth to retain a proper cutting form when the hob is subsequently re-sharpened.
In manufacturing a conventional hob, the teeth are milled and then are backed off to provide the necessary cutting clearance. Thereafter, the hob is hardened and finish ground. During the finish grinding operation, the hob is rotated relative to a rotating grinding wheel which is fed into the hob in such a manner as to cause the tip, the flanks and the root of the tooth to be shaped as Archimedean spirals. This grinding process shapes the tooth in such a manner as to enable the tooth to be later re-sharpened while maintaining the same clearance angles.
Grinding of a conventional hob involves a compromise between the clearance angle, the usable tooth length and the grinding wheel diameter. As mentioned above, high clearance angles are desirable to enable free cutting and high performance. In a conventional hob, it is important for the teeth to have a long usable length so that the teeth may be re-sharpened many times. It also is desirable to use a grinding wheel of large diameter in order to reduce wheel wear and to minimize the frequency with which the wheel must be dressed. The diameter of the grinding wheel, however, is limited due to the fact that the wheel cannot interfere with the tooth which is circumferentially adjacent the tooth being ground. If the grinding wheel diameter and the clearance angle are increased, the usable tooth length must be decreased. Thus, a compromise of these factors must be reached in designing the hob.
Re-sharpening of a gear cutting tool is time-consuming and must be performed with high precision. In order to avoid the need for a re-sharpening operation, the assignee of the present invention has developed a throw-away gear cutting tool. Such a tool is disclosed in Haug U.S. Pat. No. 4,576,527 and takes the form of a thin cutting wafer which constitutes part of a gear shaper cutter. In many cases, it is more economical to simply throw away the worn wafer and replace the worn wafer with a new wafer than to re-sharpen a conventional shaper cutter.
Consistent with this, it is contemplated that it may be feasible to simply throw away a worn hob rather than to incur the expense of a re-sharpening operation. In order to achieve this, however, it is necessary to manufacture the hob on a very economical basis so that the first cost of a replacement hob is comparatively low. Also, it is necessary that the hob be designed to achieve production rates which are sufficiently high to justify the cost of the hob. In other words, for a disposable hob to be commercially feasible, the hob must produce a total cost benefit which is greater than that realized through the use of a conventional re-sharpenable hob.